wip: isthmus operator fee

[yuwen01]

Adds the operator fee, as described in this design doc and this spec

Here, I propose that these changes be included in the Holocene hardfork instead of isthmus, like the spec says. This can easily be changed if necessary.

This PR will additionally require changes to GasPriceOracle.sol, L1Block.sol, and SystemConfig.sol in order to be usable, which @roberto-bayardo and @tynes are working on right now.